This invention relates to a power unit comprising an internal combustion engine supercharged by a turbinecompressor unit having at least one compressor and at least one turbine driving the compressor, a by-pass pipe enabling air supplied by the compressor and not absorbed by the engine to flow towards the turbine, and an auxiliary combustion chamber disposed upstream of the turbine receiving the air flowing in the by-pass pipe.
Power units of this kind are described in U.S. Pat. No. 3,988,894 assigned to the assignee of the present invention.
In simple cases, the engine by-pass pipe can be wide open. In other embodiments described in U.S. Pat. No. 3,988,894, the by-pass is provided with throttle means having a continuously variable flow cross-section and controlled substantially by the pressures upstream and downstream of the throttle means, an increase in the pressure downstream of the throttle means tending to reduce the flow cross-section of the throttle means, and an increase in the pressure upstream of the throttle means tending to increase the flow cross-section, so that the pressure difference produced by the throttle means varies directly with the pressure in the by-pass pipe upstream of the throttle means, typically in a substantially linear manner.
The invention is applicable mainly though not exclusively to diesel engines (i.e. engines in which the fuel injected into each cylinder is ignited simply by the temperature and pressure of the air in the cylinder at the moment of injection) and applies still more particularly though not exclusively to diesel engines having a high supercharging ratio (which may exceed 5) and a relatively low compression ratio (which may be less than 8). Very highly supercharged diesel engines of this kind are adapted to burn four or five times as much fuel per cycle as a similar engine which is not supercharged. The engine injection pump should be suitably dimensioned. However, in order not to increase excessively the gas temperature (inter alia the temperature of the exhaust gases), which would result in the rapid destruction of engine components, the engine must be provided not only with the air required for burning the injected fuel but also with an excess of air (up to 150% of the preceding amount) so as to maintain the temperature of the gases in the engine cylinders at an acceptable value. In other words, the richness (the ratio of the mass of the injected fuel to the mass of the air introduced into the engine) must be prevented from exceeding a limiting value.
It has already been proposed, e.g. in Zuhn U.S. Pat. No. 3,096,615, to protect a supercharged engine by limiting the rate of fuel injected per cycle in dependence on the quantity of air supplied by the compressor, i.e. substantially in dependence on the supercharging pressure. To this end, the fuel supply system is provided with flow-limiting means which usually, in the case of a mechanical supply pump, comprise means for regulating the position of the pump rack stop and sensitive to the supercharging pressure.
An arrangement of this kind, which can be called "passive" since it has only a limiting effect, actually protects the engine by preventing the fuel injection rate from rising above a value which is not dangerous for the engine, allowing for the inlet pressure, which cannot be controlled in the case of a conventional supercharged engine (in which the exhaust gases are not reheated before being introduced into the turbine). In the case of a power unit provided with an auxiliary fuel chamber of the kind described in the aforementioned U.S. Patent Specification, this problem can be solved only if the substantially constant value at which the supercharging pressure is maintained is set at a level corresponding to the most unfavourable engine operating conditions, i.e. too high for most operating conditions, resulting in the disadvantage of excessive fuel consumption in the auxiliary combustion chamber.
Furthermore, the limitation is a serious disadvantage for any supercharged engine which does not have a permanently open by-pass pipe, since its acceleration capacity is very unfavourably affected since the increase in the flow rate of fuel injected into the motor and required during acceleration is delayed by the limiting means until the supercharging pressure has increased to a high value so as to withdraw the rack stop of the fuel injection pump. In other words, the engine is slow to respond to action tending to increase the load or the speed. This disadvantage increases in proportion to the supercharging ratio under full load; in practice it becomes unacceptable in engines which must respond very quickly to a large torque requirement and which have a supercharging ratio which is greater than conventional ratios, which do not exceed 4.